The present invention relates to an auxiliary modular oven assembly for use with a gas chromatograph. The auxiliary oven assembly is comprised of a housing, an oven compartment, a heater, at least one sampling valve, at least one fixed volume sample loop, and at least one chromatograph column or column trains, wherein the column or column trains contains a material capable of causing the separation of chemical components of a chemical sample at elevated temperatures.
Refinery and chemical plant product streams are often analyzed by gas chromatography to determine what chemical components are present and their concentrations. It is often important that the analysis be performed as quickly as possible to give the operator of a process unit feedback as to the product state being produced. The longer the process unit is producing an off-spec product slate, the more money is lost.
Chromatography is a well known analytical tool. Chromatography involves the separation of a mixture of chemical compounds (solutes) into separate components. It is easier to identify (qualitate) and measure the amount (quantitate) of the various sample components by separating the sample into individual components. There are numerous chromatographic techniques and corresponding instruments. Gas chromatography is the chromatography of interest in the practice of the present invention. To be suitable for gas chromatographic analysis, a compound must have sufficient volatility with thermal stability. If all, or some of, the chemical components are in the gas or vapor phase at 400-450xc2x0 C. or below, and they do not decompose at these temperatures, the components can most likely be analyzed by gas chromatography.
A typical gas chromatography system is comprised of an oven in which is located a column containing a packing material (stationary phase) capable of causing the separation of the chemical components of the chemical stream to be analyzed when the stream is vaporized; and at least one detector capable of detecting the individual vaporized chemical components of the sample stream. The oven is typically programmable to a temperature range that will vaporize the components of the sample stream to be analyzed. In gas chromatography, the chemical stream to be analyzed is injected into a sampling valve that directs it to a fixed volume loop wherein it is swept into a carrier gas then through a chromatograph column or column train that is maintained at suitable temperatures for separation of chemical components of the sample stream. The chemical components take different times to travel the length of the column. Components having more affinity for the packing material in the column will tend to be retained in the packing, and their migration through the column will take a longer time. A detector determines the type and concentration of each chemical component that is separated in the column.
The current state of gas chromatography art can be divided into several categories. A first category includes methods that use a conventional gas chromatograph as a platform. These instruments use conventional packed columns (xe2x85x9xe2x80x3 ID), having a relatively large bore, and standard two position rotary chromatography valves. Such instruments typically require from about 15 to 65 minutes to complete a refinery gas analysis. Analysis speed is typically restricted by the speed of the valves, system dead volume, column flow rates, column efficiency, and oven temperature programming limits. The majority of conventional gas chromatograph systems in use today fall within this category.
A second category of gas chromatograph systems uses a conventional gas chromatograph as a platform but employs fast switching diaphragm valves, micro-bore capillary columns and micro-packed columns to accomplish the analysis in as little as 5 minutes. Such instruments improve analysis speed, but sacrifice sample capacity, detection levels, and flexibility.
A third category includes valved, micro gas chromatograph systems that are fast, but that have operating limits that can restrict its flexibility and use by the user. Refinery gases can be analyzed in such systems in as little as 160 seconds, but the instrument""s operating parameters cannot be modified to accommodate changes in sample matrices. For example, there is no capillary inlet splitter so the sample size cannot be changed. Also, the columns that are used are of very small bore, consequently sample capacity is limited. As a result, column overload and limited detection levels are typical. The types of detectors that are available for such instruments are very limited, thus, lack of flexibility is an issue. Service on such an instrument typically cannot be performed in the field because of the exotic nature of the device. Reliability, down time, and maintenance costs are also significant issues. Further, liquefied gas samples, before being analyzed, require special treatment. Any liquid phase sample that is injected into such an instrument can cause damage and result in expensive repairs. Further, such an instrument typically requires four separate channels to complete an analysis. This makes calibration and report generation complex. U.S. Pat. No. 4,470,832 teaches carrier gas switching in a gas chromatograph and U.S. Pat. No. 5,340,543 teaches a module gas chromatograph device both of which are incorporated herein by reference. Also, see xe2x80x9cA New Approach to Petroleum Gas Analysis By Multidimensional Gas Chromatography by Naizhong ZOU of the Department of Chemical Engineering at Penn State University, which is also incorporated herein by reference.
Therefore, there is a need in the art for gas chromatograph systems that can analyze chemical and petroleum feed and product streams in substantially less time than presently available without the disadvantages of the current state of the art.
In accordance with the present invention, there is provided a modular auxiliary oven assembly for use with a gas chromatograph, which oven assembly is comprised of a first housing having at least one inlet fluidly connected to at least one outlet, which housing has contained therein:
a) an oven which is enclosed in a second housing and containing at least one inlet that is fluidly connected to at least one inlet of the housing and at least one outlet that is fluidly connected to at least one outlet of the housing, which oven has contained therein;
i) one or more multiport chromatographic sampling valves having inlet ports and outlet ports wherein said sampling valve is capable of receiving a fluid sample in an inlet port and directing it to an outlet port, and wherein one of its inlet ports is fluidly connected to an inlet of said housing;
ii) one or more fixed volume sampling means that is either an integral part of each one or more sampling valve or is external to the sampling valve;
iii) one or more chromatograph column or column trains capable of separating chemical components of a chemical sample wherein each column has a first end and a second end, wherein its first end is fluidly connected to an outlet port of a chromatographic sampling valve and its second end is fluidly connected to the outlet of said first enclosed compartment; and
iv) at least one heater; and
b) an actuating mechanism associated with each sampling valve for switching the sampling valve position for directing the flow of fluid through the sampling valve.
In a preferred embodiment there is provided within said auxiliary oven at least one gas switching valve for switching the analytes from two or more columns to a common detector.
In another preferred embodiment there is provided a series of two or more chromatograph column or column trains.
In yet another preferred embodiment of the present invention the oven assembly is integrated with a gas chromatograph by fluidly connecting the outlet of the housing of the oven assembly to the inlet of a detector of the gas chromatograph.
In still another preferred embodiment of the present invention at least one of the chromatographic columns of the oven compartment is a column having a geometry selected to maximize the carrier gas linear velocity and sample capacity without overwhelming the column or detector.